Receiver with non-coherent matched filter

ABSTRACT

In one implementation, a receiver has a module to detect a carrier within a portion of a digital representation of a received signal. In addition, the receiver includes a module to calculate the cross-correlation between the portion of the digital representation of the received signal and a reference signal representing an expected pulse pattern. The receiver also has a module to generate an estimate of a portion of a message potentially included in the digital representation of the received signal. The receiver further includes a screening module to generate a feature vector representing the estimated message, project the feature vector into a feature space, and determine the likelihood that the digital representation of the received signal includes a message. If the digital representation of the received signal likely includes a message, the receiver includes a non-coherent matched filter to recover the message from the digital representation of the received signal.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.15/462,415 filed on Mar. 17, 2017, the disclosures of which areincorporated herein by reference.

TECHNICAL FIELD

The disclosure relates generally to receiver technology, and morespecifically to a receiver with a non-coherent matched filter.

SUMMARY

According to one implementation of the disclosure, a receiver forreceiving 1090 MHz Mode S Extended Squitter (“ES”) ADS-B messagesincludes an analog-to-digital converter configured to convert a receivedanalog signal into a digital representation of the received signal and acarrier detection module configured to determine if a spectral componentwithin a range of 1090 MHz is present within a portion of the digitalrepresentation of the received signal. In addition, the receiverincludes a cross-correlation module configured to calculate, responsiveto a determination by the carrier detection module that a spectralcomponent within the range of 1090 MHz is present within the portion ofthe digital representation of the received signal, a measure of thecross-correlation between the portion of the digital representation ofthe received signal and a reference signal representing an expectedpulse pattern for a specific portion of a 1090 MHz Mode S ES ADS-Bmessage, the calculated measure of the cross-correlation representing afirst measure of the likelihood that the digital representation of thereceived signal includes a 1090 MHz Mode S ES ADS-B message. Thecross-correlation module is further configured to determine if the firstmeasure of the likelihood that the digital representation of thereceived signal includes a 1090 MHz Mode S ES ADS-B message satisfies afirst condition. The receiver also includes a signal estimator moduleconfigured to generate, responsive to a determination that the firstmeasure satisfies the first condition, an estimate of a portion of a1090 MHz Mode S ES ADS-B message potentially included in the digitalrepresentation of the received signal corresponding to the portion ofthe digital representation of the received signal. The receiver furtherincludes a screening module. The screening module is configured togenerate a feature vector representing n≥2 features of the estimate ofthe portion of the 1090 MHz Mode S ES ADS-B potentially included in thedigital representation of the received signal, project the featurevector into a corresponding n-dimensional feature space, determine,based on the projection of the feature vector into the feature space, asecond measure of the likelihood that the digital representation of thereceived signal includes a 1090 MHz Mode S ES ADS-B message, anddetermine if the second measure of the likelihood that the digitalrepresentation of the received signal includes a 1090 MHz Mode S ESADS-B message satisfies a second condition. If the second measure of thelikelihood that the digital representation of the received signalincludes a 1090 MHz Mode S ES ADS-B message suggests that the digitalrepresentation of the received signal includes a 1090 MHz Mode S ESADS-B message, the receiver has a non-coherent matched filter configuredto recover, from the digital representation of the received signal, a1090 MHz Mode S ES ADS-B message.

Other features of the present disclosure will be apparent in view of thefollowing detailed description of the disclosure and the accompanyingdrawings. Implementations described herein, including theabove-described implementations, may include a method or process, asystem, or computer-readable program code embodied on computer-readablemedia.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present disclosure, referencenow is made to the following description taken in connection with theaccompanying drawings.

FIG. 1 is a high level block diagram of an example of a space-basedADS-B system in accordance with the present disclosure.

FIG. 2 is a flow chart of a method for receiving 1090 MHz Mode S ESADS-B messages in accordance with a non-limiting implementation of thepresent disclosure.

FIG. 3 is a functional block diagram of one example of a receiver forreceiving 1090 MHz Mode S ES ADS-B messages in accordance with anon-limiting implementation of the present disclosure.

FIG. 4 is a functional block diagram of one example of a receiver forreceiving 1090 MHz Mode S ES ADS-B messages in accordance with anon-limiting implementation of the present disclosure.

FIG. 5 is a functional block diagram of one example of a multi-layersignal screening test block in accordance with a non-limitingimplementation of the present disclosure.

FIG. 6 is a functional block diagram of one example of a minimum meansquare error signal estimator in accordance with a non-limitingimplementation of the present disclosure.

FIG. 7 is a functional block diagram of one example of a moduleconfigured to calculate the residual phase error of a signal inaccordance with a non-limiting implementation of the present disclosure.

FIG. 8 is a functional block diagram of one example of a moduleconfigured to calculate a measure of the amplitude consistency of pulseswithin a signal and a measure of the phase consistency of the signal inaccordance with a non-limiting implementation of the present disclosure.

FIG. 9 is a functional block diagram of one example of a patternrecognition module in accordance with a non-limiting implementation ofthe present disclosure.

FIG. 10 is a functional block diagram of one example of a re-triggeringtest module in accordance with a non-limiting implementation of thepresent disclosure.

FIG. 11 is a functional block diagram of one example of a non-coherentmatched filter in accordance with a non-limiting implementation of thepresent disclosure.

FIG. 12 is a functional block diagram of one example of a carrierfrequency estimator in accordance with a non-limiting implementation ofthe present disclosure.

DETAILED DESCRIPTION

Traditionally, air traffic control, aircraft surveillance, and flightpath management services have relied on ground-based radar stations andsurveillance data processing systems. These systems rely onaircraft-based radio transmitters and terrestrial interrogation andreceiving stations to implement systems, such as, for example, primarysurveillance radar (“PSR”), secondary surveillance radar (“SSR”), and/ormode select (“Mode S”) radar, for communicating aircraft position andmonitoring information to local ground stations. The informationreceived at the local ground stations is then relayed to regional orglobal aircraft monitoring systems. Such conventional radar-basedsystems for use in air traffic control, aircraft surveillance, andflight path management services are limited to use in regions in whichthe appropriate ground infrastructure exists to interrogate and receivemessages from aircraft. Consequently, vast areas of the world's airspace(e.g., over the oceans and poles, remote and/or mountainous regions,etc.) are not monitored by conventional, terrestrial radar-basedsystems.

Recently, modernization efforts have been launched to replaceradar-based air traffic control, aircraft surveillance, and flightmanagement systems with more advanced automatic dependentsurveillance-broadcast (“ADS-B”) based systems. In an ADS-B-basedsystem, an aircraft determines its position using a satellite-basednavigation system (e.g., the Global Positioning System (“GPS”)) andperiodically broadcasts its position and, in some cases, otherinformation (e.g., velocity, time, and/or intent, among otherinformation), thereby enabling the aircraft to be tracked. ADS-B-basedsystems may utilize different data links and formats for broadcastingADS-B messages. 1090 MHz Mode S ES is an example of one such data linkwhich has been adopted in many jurisdictions. For example, in the UnitedStates, the Federal Aviation Administration (“FAA”) has mandated 1090MHz Mode S ES for use by air carrier and private or commercial operatorsof high-performance aircraft by 2020. Like traditional radar-basedsystems, ADS-B-based systems require appropriate infrastructure forreceiving ADS-B messages broadcast by aircraft. As a result, even asnumerous jurisdictions transition to terrestrial, ADS-B-based systems,air traffic in vast airspaces remains unmonitored where there is no suchterrestrial infrastructure.

To address this limitation of terrestrial ADS-B systems, satellite-basedreceivers can be used to receive ADS-B messages broadcast by aircraft,and such ABS-B messages then can be relayed back down to earth terminalsor other terrestrial communications infrastructure for transmission toand use by air traffic control, aircraft surveillance, and flight pathmanagement services.

For example, and with reference to FIG. 1, a high-level block diagram ofone example of a space-based ADS-B system 100 is illustrated inaccordance with the present disclosure. System 100 includes satellite 10in communication with (and part of) satellite network 20, and aircraft70. In some implementations, satellite network 20, including satellite10, may be a low-Earth orbit (“LEO”) constellation of cross-linkedcommunications satellites. As illustrated in FIG. 1, terrestrial ADS-Bground station 60, air traffic management system 40 and satellitecommunication network earth terminal 30 are located on Earth 80'ssurface.

Aircraft 70 carries an on-board ADS-B transponder 72 that broadcastsADS-B messages containing flight status and tracking information.Satellite 10 carries ADS-B receiver 12 to receive ABS-B messagesbroadcast by aircraft 70 and other aircraft. In some implementations,multiple or all of the satellites in satellite network 20 may carryADS-B receivers to receive ADS-B messages broadcast by aircraft.Messages received at receiver 12 are relayed through satellite network20 to satellite communication network earth terminal 30 and ultimatelyto air traffic management system 40 through terrestrial network 50. Theair traffic management system 40 may receive aircraft status informationfrom various aircraft and provide additional services such as airtraffic control and scheduling or pass appropriate information along toother systems or entities.

In certain implementations, satellite network 20 may have a primarymission other than receiving ADS-B messages broadcast by aircraft. Forexample, in some implementations, satellite network 20 may be a LEO,mobile satellite communications constellation. In such implementations,ADS-B receivers like ADS-B receiver 12 may be hosted on satellites 10 ofsatellite network 20 as hosted or secondary payloads that may beconsidered secondary to the primary mission of the satellite network 20.Consequently, such ADS-B receivers when operated as hosted payloads maybe constrained by certain limitations, such as, for example, arelatively low maximum weight (e.g., ˜50-60 kilograms/payload) and arelatively low power budget (e.g., maximum average power per orbit of 70watts) so as not to take away from the primary mission of the satellitenetwork 20.

Terrestrial ADS-B ground station 60 provides aircraft surveillancecoverage for a relatively small portion of airspace, for example,limited to aircraft within line of sight of ground station 60. Even ifterrestrial ADS-B ground stations like ground station 60 are widelydispersed across land regions, large swaths of airspace (e.g., over theoceans) will remain uncovered. Meanwhile, a spaced-based ADS-B system100 utilizing a satellite network like satellite network 20 may providecoverage of airspace over both land and sea regions without beinglimited to areas where ground-based surveillance infrastructure has beeninstalled. Thus, a space-based ADS-B system may be preferable (or avaluable supplement) to terrestrial approaches.

However, implementing a spaced-based ADS-B system, such as, for example,system 100 presents a number of challenges. For instance, as illustratedin FIG. 1, the distance between transponder 72 on an aircraft 70 andterrestrial ADS-B ground station 60 may be much shorter than thedistance between a transponder 72 on an aircraft and a satellite 12,which may be, for example, in low-Earth orbit. For example, a typicalterrestrial ADS-B ground station like ADS-B ground station 60 may have atypical maximum range of approximately 150 miles whereas a satellite inlow-Earth orbit may orbit the Earth at an altitude as high asapproximately 1,243 miles. This significant difference in propagationdistance for ADS-B messages may make successful detection and receptionof ADS-B messages by a satellite-based ADS-B receiver much moredifficult than by a terrestrial-based ADS-B receiver. Furthermore,satellites in low-Earth orbit may orbit the Earth at speeds upwards of17,000 miles per hour, resulting in Doppler shifts that add additionalcomplications to successfully receiving ADS-B messages. Moreover, giventhe wider coverage area provided by a satellite as compared to aterrestrial ground station, a satellite-based ADS-B receiver may beexposed to a much higher volume of ADS-B messages than aterrestrial-based ADS-B receiver. This increased volume of receivedADS-B messages only compounds the difficulty of successfully detectingand receiving ADS-B messages with a satellite-based ADS-B receiverrelative to a terrestrial-based ADS-B receiver.

For terrestrial ADS-B receivers, the physical channel may be assumed toexhibit the characteristics of an interference channel, whereinterference and/or noise may result from interference or noise in thephysical channel itself as well as interference or noise in the antennaor receiver. Interference and/or noise in the physical channel mayresult from multiple aircraft within range of a terrestrial ADS-Breceiver broadcasting ADS-B messages in a fashion that is uncoordinatedin time. As a result, ADS-B messages that arrive at the terrestrialreceiver may interfere and/or overlap with one another. ADS-B messagesthat interfere and/or overlap with a desired ADS-B message may bereferred to as (or may be one component of) false replies unsynchronizedwith interrogator transmissions or, alternatively, false repliesunsynchronized in time (“FRUIT”).

Other communications protocols that share the 1090 MHz band with ADS-Balso may contribute interference and be a source of FRUIT. For example,aircraft implementing secondary surveillance radar (“SSR”) like Mode A,Mode C, or Mode S, may respond to interrogating SSR messages in the 1090MHz band, potentially creating interference for ADS-B messages. Othertransmitters within range of a terrestrial ADS-B receiver transmittingin neighboring or nearby frequency bands also may generate interferenceor contribute to noise in the physical channel. Appropriately dealingwith FRUIT and other interference/noise, particularly for airspaces witha high density of air traffic, may be one challenge faced by aterrestrial ADS-B receiver.

Another challenge faced by a terrestrial ADS-B receiver may be theso-called near-far problem where a signal received at the ADS-B receiverfrom a relatively nearby aircraft is significantly stronger than asignal received at the ADS-B receiver at the same time from an aircraftthat is relatively far from the ADS-B receiver.

A space-based ADS-B receiver may face the same challenges as aterrestrial ADS-B receiver. In addition, as discussed above, aspace-based ADS-B receiver may face additional challenges that may beeven more imposing than those faced by a terrestrial ADS-B receiver dueto, for example, but not limited to, the significantly greaterpropagation distance between aircraft broadcasting ADS-B messages andthe space-based ADS-B receiver, significant Doppler shifts due to theorbital velocity of a satellite, and/or the significantly greater volumeof ADS-B messages within the coverage area of a space-based ADS-Breceiver relative to a terrestrial ADS-B receiver. Different from thephysical channel for a terrestrial ADS-B receiver, the physical channelfor a space-based ADS-B receiver may be dominated by noise.Consequently, for a space-based ADS-B receiver, the physical channel maybe assumed to exhibit the characteristics of an additive white Gaussiannoise (“AWGN”) channel with additional interference channelcharacteristics. Because of this additional noise, the signal-to-noiseratio (“SNR”) for an ADS-B message received by a space-based ADS-Breceiver typically will be much lower than the SNR for an ADS-B messagereceived by a terrestrial ADS-B receiver. Existing terrestrial ADS-Breceivers have been shown to be incapable of achieving satisfactoryperformance for space-based operation due to the lower SNR of ADS-Bmessages to be received in space. In fact, existing terrestrial ADS-Breceivers are believed to require an SNR that is ˜7-10 dB greater thanthe SNR of an ADS-B message to be received in low-Earth orbit in orderto achieve satisfactory performance. For example, a space-based ADS-Breceiver in low-Earth orbit may be designed to achieve performance ofnot worse than 10% bit (or message) error rate at an energy per bit tonoise power spectral density ratio (“E_(b)/N_(o)”) no more than or equalto 10 dB, whereas existing terrestrial ADS-B receivers typically achieve10% bit error rate at E_(b)/N_(o) greater than or equal to approximately18 dB.

The teachings of the present disclosure present receiver designs andtechniques for receiving ABS-B messages transmitted by aircraft,particularly 1090 MHz Mode S ES ADS-B messages, from space, for example,on one or more low-Earth orbit satellites. As described in greaterdetail below, the expected pulse pattern of the preamble and the firstfew bit periods of the data block for a 1090 MHz Mode S ES ADS-B messagemay be used to process a signal received by a space-based ADS-B receiverto determine if it is likely to include an ADS-B message. A signal thatis determined to be likely to include an ADS-B message then is subjectedto further screening, for example, to confirm the presence of an ADS-Bmessage, demodulate the ADS-B message from the received signal, and/orperform error detection and correction of the bits of the demodulatedmessage.

The structure and format of a 1090 MHz Mode S ES ADS-B message isdefined in the Radio Technical Commission for Aeronautics' (“RTCA”)“DO-260B Minimum Operational Performance Standards for 1090 MHz ExtendedSquitter Automatic Dependent Surveillance-Broadcast (ADS-B) and TrafficInformation Services-Broadcast (TIS-B).” 1090 MHz Mode S ES ADS-Bmessages are pulse position modulated and have 1 μsec bit periods. Apulse transmitted in the first half of the bit period represents thevalue of the bit as a “1,” while a pulse transmitted in the second halfof the bit period represents the value of the bit as a “0.” All 1090 MHzMode S ES ADS-B messages have a specific 8 μsec preamble that identifiessuch messages as 1090 MHz Mode S ES ADS-B messages followed by a 112μsec (or 112 bit) data block. The preamble that identifies a message asa 1090 MHz Mode S ES ADS-B message includes a first pulse from 0.0-0.5μsec, a second pulse from 1.0-1.5 μsec, a third pulse from 3.5-4.0 μsec,and a fourth pulse from 4.5-5.0 μsec.

While detecting the presence of pulses corresponding to the pattern forthe preamble for a 1090 MHz Mode S ES ADS-B message may be helpful inidentifying the potential presence of an ADS-B message in the receivedsignal, it may be difficult to detect the preamble pulse pattern, whichonly includes four pulses over the course of 8 bit periods (or 8 μsec),in the presence of noise, particularly for a space-based ADS-B receiverwhere the pulses may have very little energy and the SNR is very low dueto the propagation distance between an aircraft and the space-basedADS-B receiver. Fortunately, however, subsequent bits of a 1090 MHz ModeS ES ADS-B message also may have an expected pulse pattern and beconsidered in an effort to detect the potential presence of a Mode S1090 ES ADS-B message in a received signal. Specifically, for a Mode S1090 ES ADS-B message, the first five bits of the data block followingthe preamble identify the downlink format (“DF”) for the ADS-B message,and, in some implementations of a space-based ADS-B receiver, only ADS-Bmessages having specific downlink formats may be relevant. For example,in some implementations, only ADS-B messages having downlink formatsDF=17, DF=18, or DF=19, which share three bits, may be relevant.Consequently, in addition to the preamble, the first 5 bits of areceived signal also may be helpful in identifying the potentialpresence of an ADS-B message within a received signal, which may improveaccuracy in the presence of noise relative to only using the preamble,particularly in the case of a space-based ADS-B receiver where thepulses have very little energy and the SNR is very low due to thepropagation distance between an aircraft and the space-based ADS-Breceiver.

In some implementations, the cross-correlation between samples of aportion of a received signal (e.g., 13 μsec (or bit periods) of areceived signal) and a reference signal representing a first portion(e.g., the first 13 μsec (or bit periods)) of an ADS-B signal may becalculated to determine a measure of the likelihood that the receivedsignal includes an ADS-B message. If the result of calculating thecross-correlation of the samples of the portion of the received signaland the reference signal exceeds a defined threshold value, thenadditional screening may be performed to determine if the receivedsignal does, in fact, include an ADS-B message.

For instance, constant false alarm rate (“CFAR”) detection processingmay be applied to a portion of the received signal to further assess thelikelihood that the received signal includes an ADS-B message, forexample, by determining if the power in the received signal exceeds athreshold level above which the received signal may be considered likelyto include an ADS-B message. In some implementations, the threshold maybe set in an effort to achieve a desired probability of false alarm (orfalse alarm rate or time between false alarms), hence the name constantfalse alarm rate detection. In some implementations, the backgroundagainst which ADS-B messages are to be detected may be assumed to beconstant with time and space, such that a fixed threshold level may bechosen to achieve the desired probability of false alarm, which will bea function of the probability density function of the noise and/orsignal-to-noise ratio. In alternative implementations, noise levels maybe assumed to change both spatially and temporally, in which case achanging threshold may be used, where the threshold level is raised andlowered to maintain a desired probability of false alarm. Variousdifferent algorithms may be used to adaptively select the thresholdlevel, for example, based on the statistics of the background in whichthe ADS-B messages are to be detected.

If the result of CFAR detection processing suggests that the receivedsignal includes an ADS-B message, a minimum mean square error (MMSE)estimator may be used to generate an estimate of the ADS-B message, or aportion thereof, and various screening techniques may be employed on theADS-B message estimate to further assess the likelihood that thereceived signal includes an ADS-B message. In some implementations, afeature vector representing some number n different features of theADS-B message estimate may be projected into an n-dimensional featurespace and the distance between the projection and a region (e.g., acluster) or a position in the n-dimensional feature space where ADS-Bmessages would be expected to appear may be calculated. If the distanceis less than some defined threshold value, a determination may be madethat the received signal is likely to include an ADS-B message. Forexample, in some implementations, a three-dimensional feature vectorrepresenting three features of the ADS-B message estimate may begenerated and projected into a three-dimensional feature space. In suchimplementations, the three features may represent (1) a measure of theconsistency of the pulse amplitude across the ADS-B message estimate,(2) a measure of phase consistency across the ADS-B message, and (3) ameasure of the residual phase error of the ADS-B message estimate. Ifthe projection of the feature vector in the 3-dimensional feature spaceis within a defined threshold distance of a region or position in thethree-dimensional feature space where ADS-B messages would be expectedto appear, a determination may be made that the received signal includesan ADS-B message. The received signal then may be input to anon-coherent matched filter to recover the ADS-B message from thereceived signal.

With reference to FIG. 2, a flow chart of a method 200 for receiving1090 MHz Mode S ES ADS-B messages is illustrated in accordance with anon-limiting implementation of the present disclosure. In someimplementations, method 200 may be performed by a receiver for receiving1090 MHz Mode S ES ADS-B messages that is implemented in one or morelogic modules of a field programmable gate array (“FPGA”). FIG. 3 is afunctional block diagram of one example of a receiver 300 (e.g., amulti-channel receiver) for receiving 1090 MHz Mode S ES ADS-B messagesthat may perform method 200. In some implementations, each blockrepresented in FIG. 3 may correspond to a specific logic module of anFPGA. As illustrated in FIG. 3, receiver 300 includes four channels302(1)-302(4). Consequently, receiver 300 is capable of processing fourreceived signals concurrently in parallel. Similar to FIG. 3, FIG. 4 isa functional block diagram of one example of a receiver 400 forreceiving 1090 MHz Mode S ES ADS-B messages that may perform method 200.As with receiver 300 illustrated in FIG. 3, each block represented inFIG. 4 may correspond to a specific logic module of an FPGA. Althoughthe receiver 400 illustrated in FIG. 4 is shown as having only a singlechannel, implementations of receiver 400 may include multiple channels(e.g., 4 channels like the receiver 300 illustrated in FIG. 3).

For the purposes of the present disclosure, the method 200 illustratedin FIG. 2 will be described with reference to the receiver 300 of FIG.3, and, in particular, with reference to a single channel 302 ofreceiver 300. However, the method 200 illustrated in FIG. 2 may beperformed by a variety of different hardware architectures. For example,in contrast to the FPGA implementation of receiver 300, in someimplementations, the method 200 illustrated in FIG. 2 may be implementedby one or more microprocessors executing machine-readable instructionsstored in electronic memory.

Referring now to FIG. 2, at step 210, a received analog signal (or atleast a portion of a received analog signal) is converted into a digitalrepresentation of the received analog signal. In some implementations,at least fifteen samples per 1090 MHz Mode S ES ADS-B message bit period(e.g., 15 samples per μsec) may be taken in converting the receivedanalog signal to digital.

At step 220, the digital representation of the received signal may beprocessed to determine if a 1090 MHz spectral component is present. Forexample, block 304 of receiver 300 of FIG. 3 may perform a fast Fouriertransform (FFT) on the digital representation of the received signal (ora portion of the digital representation of the received signal) toconvert the digital representation of the received signal (or portionthereof) into the frequency domain. Block 306 then may analyze thefrequency domain representation of the digital representation of thereceived signal (or portion thereof) to determine if a 1090 MHz spectralcomponent is present. In some implementations, a 1090 MHz spectralcomponent may be determined to be present if a spectral component withinsome defined range of 1090 MHz is present (e.g., +1-1.05 MHz).Additionally or alternatively, in some implementations block 306 maydetect the phase of the envelope of the digital representation of thereceived signal and remove the phase to facilitate detection of thecarrier. Furthermore, in some implementations, block 306 also may shift(or attempt to shift) the frequency of the digital representation of thereceived signal closer to 1090 MHz (e.g., to account for Doppler shift).Moreover, in some implementations, block 306 may include a carrierfrequency estimator, such as, for example, a preliminary quadraticfrequency estimator, one particular example of which is illustrated inFIG. 12. In some implementations, 8 μsec (e.g., corresponding to thelength of the preamble of a 1090 MHz Mode S ES ADS-B message) or 13 μsec(e.g., corresponding to the length of the preamble and the first 5 bitperiods of the data block of a 1090 MHz Mode S ES ADS-B message) may beprocessed to determine if a 1090 MHz spectral component is present.

Referring again to FIG. 2, if a 1090 MHz spectral component is notdetermined to be present, the digital representation of the receivedsignal (or portion thereof) may be discarded and the process 200 mayreturn to step 210. Alternatively, if a 1090 MHz spectral component isdetermined to be present, the process 200 may proceed to step 230.

At step 230, the cross-correlation between the digital representation ofthe received signal (or a portion thereof) and a reference signalrepresenting an expected pulse pattern is calculated. For example, block308 of receiver 300 of FIG. 3 may calculate the cross-correlation (orthe complex cross-correlation) between an 8 μsec or 13 μsec segment ofthe digital representation of the received signal and a reference signalrepresenting the expected pulse pattern of the preamble of a 1090 MHzMode S ES ADS-B message (e.g., in the case of an 8 μsec segment) or theexpected pulse pattern of the preamble and the first 5 bit periods ofthe data block of a 1090 MHz Mode S ES ADS-B message (e.g., in the caseof a 13 μsec segment). In some implementations, the reference signal mayrepresent the perfect theoretical expected pulse pattern (e.g., basebandpulse pattern) in the absence of any noise.

Thereafter, at 240, a determination may be made as to whether it islikely that the digital representation of the received signal includes a1090 MHz Mode S ES ADS-B message based on the calculatedcross-correlation. For example, block 308 of receiver 300 of FIG. 3 maycompare a measure of the cross-correlation to a threshold value anddetermine that it is unlikely that the digital representation of thereceived signal includes a 1090 MHz Mode S ES ADS-B message if themeasure is less than a threshold value or that it is likely that thedigital representation of the received signal includes a 1090 MHz Mode SES ADS-B message if the measure is greater than the threshold value.

In some implementations, additional information may be taken intoaccount at step 240 when determining the likelihood that the digitalrepresentation of the received signal includes a 1090 MHz Mode S ESADS-B message. For example, in some implementations, block 310 ofreceiver 300 of FIG. 3 may perform CFAR detection processing on thedigital representation of the received signal (or portion thereof) aspart of determining the likelihood that the digital representation ofthe received signal includes a 1090 MHz Mode S ES ADS-B message. In suchimplementations, a determination may be made that it is likely that thedigital representation of the received signal includes a 1090 MHz Mode SES ADS-B message if the measure of the cross-correlation of the digitalrepresentation of the received signal (or portion thereof) and thereference signal exceeds a first threshold value and the CFAR detectionprocessing reveals that the power in the received signal exceeds asecond threshold level, which may be adjusted dynamically over time inan effort to maintain a desired rate of false alert detection.Otherwise, a determination may be made that it is unlikely that thedigital representation of the received signal includes a 1090 MHz Mode SES ADS-B message.

Referring again to FIG. 2, if it is determined that it is unlikely thatthe digital representation of the received signal includes a 1090 MHzMode S ES ADS-B message, the digital representation of the receivedsignal (or portion thereof) may be discarded, and process 200 may returnto step 210. Alternatively, if it is determined that it is likely thatthe digital representation of the received signal includes a 1090 MHzMode S ES ADS-B message, process 200 may proceed to step 250.

At step 250, additional multi-layer screening of the digitalrepresentation of the received signal is performed to further assess thelikelihood that the digital representation of the received signalincludes a 1090 MHz Mode S ES ADS-B message. For example, multi-layersignal screening test block 312 of receiver 300 of FIG. 3, oneparticular example of which is illustrated in greater detail in FIG. 5,may perform various different multi-layer screening techniques tofurther assess the likelihood that the digital representation of thesignal includes a 1090 MHz Mode S ES ADS-B message.

In some implementations, and as illustrated in steps 255, 260, 265, and270 of the example process 200 of FIG. 2 and described in greater detailbelow, such multi-layer screening may include: (i) generating anestimate of a 1090 MHz Mode S ES ADS-B message (or an estimate of aportion of the 1090 MHz Mode S ES ADS-B message) potentially included inthe digital representation of the received signal from the digitalrepresentation of the received signal (or from a portion of the digitalrepresentation of the received signal) (255); (ii) generating a featurevector representing n≥2 features of the estimated 1090 MHz Mode S ESADS-B message (or portion thereof) (260); (iii) projecting the featurevector into an n-dimensional feature space (265); and (iv) comparing theprojection of the feature vector to a cluster of points in the featurespace representing sample 1090 MHz Mode S ES ADS-B messages (270).

Referring specifically to step 255, in some implementations, an estimateof a 1090 MHz Mode S ES ADS-B message (or a portion thereof (e.g., thepreamble or the first 13 bit periods) potentially included in thedigital representation of the received signal may be generated from thedigital representation of the received signal using a minimum meansquare error (“MMSE”) estimator. For example, in some implementations,an MMSE estimator may be used to generate an estimate of an 8 μsecportion of a 1090 MHz Mode S ES ADS-B message potentially included inthe digital representation of the received signal for the purpose ofcomparing certain features of the estimate to expected or representativefeatures of the preamble of a 1090 MHz Mode S ES ADS-B message.Alternatively, in other implementations, an MMSE estimator may be usedto generate an estimate of a 13 μsec portion of a 1090 MHz Mode S ESADS-B message potentially included in the digital representation of thereceived signal for the purpose of comparing certain features of theestimate to expected or representative features of the preamble and thefirst 5 bit periods of a 1090 MHz Mode S ES ADS-B message.

Referring now to FIG. 5, in some implementations, the estimate of the1090 MHz Mode S ES ADS-B message (or a portion thereof) may be generatedby MMSE estimator block 502 of the example multi-layer screening testblock 312, one particular example of which is illustrated in greaterdetail in FIG. 6.

Referring again to FIG. 2, at step 260, a feature vector representingsome number n different features of the estimate of the 1090 MHz Mode SES ADS-B message (or portion thereof) may be generated. Depending on theimplementation, various different features of the estimate may berepresented in the feature vector. In some implementations, one or moreof a measure of the amplitude consistency of pulses within the estimate,a measure of the phase consistency of the estimate, and a measure of theresidual phase error of the estimate may be calculated and representedin the feature vector. For example, referring to FIG. 5, in someimplementations, a measure of the residual phase error of the estimatemay be calculated by the residual phase metric block 504 of the examplemulti-layer screening test block 312, one particular example of which isillustrated in greater detail in FIG. 7. Additionally or alternatively,in some implementations a measure of the amplitude consistency of pulseswithin the estimate and/or a measure of the phase consistency of theestimate may be calculated by the innovations metric block 506 of theexample multi-layer screening test block 312, one particular example ofwhich is illustrated in greater detail in FIG. 8.

Referring again to FIG. 2, at step 265 the feature vector may beprojected into an n-dimensional feature space where each individualdimension of the feature space represents one of the featuresrepresented in the feature vector. Thereafter, at step 270, theprojection of the feature vector of the estimate in the feature space iscompared to a cluster of points in the feature space representing sample1090 MHz Mode S ES ADS-B messages (or, if the estimate represents aportion of a 1090 MHz Mode S ES ADS-B message (e.g., the first 8 μsec or13 μsec), corresponding portions of sample ADS-B messages).

At step 280, another determination is made as to the likelihood that thedigital representation of the received signal includes a 1090 MHz Mode SES ADS-B message, this time based on the results of the multi-layerscreening of the digital representation of the received signal (orportion thereof). In the particular example illustrated in FIG. 2, wherethe multi-layer screening involves (i) generating an estimate of a 1090MHz Mode S ES ADS-B message (or an estimate of a portion of a 1090 MHzMode S ES ADS-B message) potentially included in the digitalrepresentation of the received signal from the digital representation ofthe received signal (or from a portion of the digital representation ofthe received signal) (255), (ii) generating a feature vectorrepresenting the estimate (260), (iii) projecting the feature vectorinto a feature space (265), and (iv) comparing the projection of thefeature vector to a cluster of points in the feature space representingsample 1090 MHz Mode S ES ADS-B messages (or portions thereof) (270),the determination of the likelihood that the digital representation ofthe received signal includes a 1090 MHz Mode S ES ADS-B message may bebased on the comparison of the projection of the feature vector to thecluster of samples.

For example, the comparison of the projection of the feature vector tothe cluster of samples may involve calculating some measure of thedistance between the projection of the feature vector and the cluster ofsamples in the feature space, and the determination of the likelihoodthat the digital representation of the received signal includes a 1090MHz Mode S ES ADS-B message may be based on the calculated distance. Forexample, if the distance is less than some predefined threshold value,it may be determined that it is likely that the digital representationof the received signal includes a 1090 MHz Mode S ES ADS-B message. Incontrast, if the distance is greater than the predefined thresholdvalue, it may be determined that it is unlikely that the digitalrepresentation of the received signal includes a 1090 MHz Mode S ESADS-B message. In some particular implementations where the comparisonof the projection of the feature vector to the cluster of samplesinvolves calculating a measure of the distance between the projection ofthe feature vector and the cluster of samples in the feature space, themeasure of the distance may be calculated by calculating the Mahalanobisdistance between the projection of the feature vector and a distributionof the samples in the feature space.

Referring now to FIG. 5, in the particular example implementationdescribed above where the multi-layer screening of the digitalrepresentation of the received signal involves generating an estimate ofa 1090 MHz Mode S ES ADS-B message (or an estimate of a portion of a1090 MHz Mode S ES ADS-B message) potentially included in the digitalrepresentation of the received signal from the digital representation ofthe received signal (or from a portion of the digital representation ofthe received signal), generating a feature vector representing theresidual phase error of the estimate, the amplitude consistency ofpulses within the estimate, and the phase consistency of the estimate,projecting the feature vector into a corresponding 3-dimensional featurespace, and determining the likelihood that the digital representation ofthe received signal includes a 1090 MHz Mode S ES ADS-B message based onthe distance between the projection of the feature vector and a clusterof points representing sample 1090 MHz Mode S ES ADS-B messages (orportions thereof), pattern recognition module 508 of multi-layerscreening module 312, one particular example of which is represented ingreater detail in FIG. 9, generates the feature vector, projects thefeature vector into the feature space, and calculates the distancebetween the projection of the feature vector and the cluster of samples.

In some implementations, pattern recognition module 508 may beimplemented in circuitry that performs processing that is functionallyequivalent (or functionally similar) to the functions of generating thefeature vector, projecting the feature vector into the feature space,and calculating the distance between the projection of the featurevector and the cluster of samples. Additionally or alternatively, insome implementations, pattern recognition module 508 may perform thefunctions (or functional equivalents) of generating the feature vector,projecting the feature vector into the feature space, and calculatingthe distance between the projection of the feature vector and thecluster of samples in one dimension in a manner that approximatesperforming such functions in n dimensions.

In some implementations, based on the estimate of the 1090 MHz Mode S ESADS-B message (or portion thereof) generated by multi-layer signalscreening test block 312, multi-layer signal screening test block 312may generate, among other output, an indication of a coarse estimate ofthe frequency of carrier pulses in the digital representation of thereceived signal, a bit_sync output that provides timing informationabout the digital representation of the received signal (e.g.,suggesting where bit transitions occur in the digital representation ofthe received signal to facilitate the later sampling of bits from theoutput of the coherent matched filter), and/or a msg_sync output thatprovides an indication of the offset to the first data bit in thedigital representation of the received signal (e.g., to facilitatesubsequent processing of the actual message and not the preamble).

In some implementations, re-triggering testing may be performed whilethe digital representation of the received signal (or portion thereof)is being processed by multi-layer signal screening test block 312. Forexample, re-triggering test module 316 of receiver 300 of FIG. 3, oneexample of a particular implementation of which is illustrated in moredetail in FIG. 10, may perform such re-triggering testing. The purposeof this re-triggering testing may be to determine if upcoming samples ofthe received signal are more likely to include an actual 1090 MHz Mode SES ADS-B message (or portion thereof) than the digital representation ofthe received signal (or portion thereof) currently being processed bymulti-layer signal screening test block 312 and, if so, to focus on theupcoming samples of the received signal determined to be more likely toinclude an actual 1090 MHz Mode S ES ADS-B message than the samplescurrently being processed.

Referring again to FIG. 2, if a determination is made that it isunlikely that the digital representation of the received signal includesa 1090 MHz Mode S ES ADS-B message, the process 200 returns to step 210.Alternatively, if a determination is made that it is likely that thedigital representation of the received signal includes a 1090 MHz Mode SES ADS-B message, the process 200 may proceed to step 290 where anon-coherent matched filter is used to recover the 1090 MHz Mode S ESADS-B message from the digital representation of the received signal.

For example, non-coherent matched filter module 314 of receiver 300 ofFIG. 3, one particular example of which is illustrated in greater detailin FIG. 11, may process the digital representation of the receivedsignal to recover the 1090 MHz Mode S ES ADS-B message from the digitalrepresentation of the received signal. Non-coherent matched filtermodule 314 may be said to include a matched filter because the filter ismatched to the 0.5 μsec 1090 MHz expected pulse form of a 1090 MHz ModeS ES ADS-B message. Stated differently, the impulse response of thematched filter may be a 0.5 μsec 1090 MHz pulse matched to the expectedpulse form of a 1090 MHz Mode S ES ADS-B message. For example, in someimplementations, the matched filter may include a box-car filter. Moreparticularly, where the received message is sampled at a rate of 15samples per μsec, non-coherent matched filter module 314 may include abox-car filter with 15 coefficients with the first seven coefficientshaving weights of 1 and the last eight coefficients having weights of 0.Furthermore, non-coherent matched filter module 314 may be said toinclude a non-coherent filter because the filter included innon-coherent matched filter module 314 recovers the 1090 MHz Mode S ESADS-B message from the digital representation of the received signalwithout a priori knowledge of the phase of the 1090 MHz Mode S ES ADS-Bmessage.

Non-coherent receiver designs such as the implementations describedherein may have various advantages and/or differences relative tocoherent designs. For example, a non-coherent receiver may handle signalvariations, including phase variations, better than a coherent receiver.Additionally or alternatively, a non-coherent receiver that does notattempt to shift the frequency of carrier pulses in a received signaland/or that does not attempt to align the phase of carrier pulses in thereceived signal with a desired phase may be simpler, and, thus,potentially easier, cheaper, and/or smaller to implement than a coherentfilter that does attempt to shift the frequency of carrier pulses in areceived signal and/or that does attempt to align the phase of carrierpulses in the received signal.

In implementations in which multi-layer signal screening test block 312generates a bit_sync output that provides timing information about thedigital representation of the received signal, non-coherent matchedfilter module 314 may receive the bit_sync output from the multi-layersignal screening test block 312 as input and use the bit_sync input tocoordinate the timing of the sampling of the output of the non-coherentmatched filter.

After the 1090 MHz Mode S ES ADS-B message has been recovered from thereceived signal, the actual bits of the message still need to bedemodulated. For example, determinations need to be made as to whethersamples of the recovered message correspond to 1s or 0s. Therefore, therecovered 1090 MHz Mode S ES ADS-B message output by non-coherentmatched filter module 314 may be processed by one or more of bothsoft-decision decoder 318 and hard-decision decoder 320 to demodulatethe actual bits of the message. Additionally or alternatively, in someimplementations, after the bits of the ADS-B message have beendemodulated, the bits of the ADS-B message may be processed by an errordetection and correction module 322 in an effort to detect and correctany errors in the decoded bits (e.g., due to noise or other impairmentsduring the transmission or processing of the received signal).

Thereafter, the recovered ADS-B message may be output by the receiver300, for example, for transmission to elsewhere in the system. Forexample, in implementations where receiver 300 is hosted on a satellite,the recovered ADS-B message may be output by the receiver 300 fortransmission by the satellite to a terrestrial Earth terminal, either bythe satellite directly or through a network of satellites, such as, forexample, the network 20 of satellites illustrated in FIG. 1.

Aspects of the present disclosure may be implemented entirely inhardware, entirely in software (including firmware, resident software,micro-code, etc.) or in combinations of software and hardware that mayall generally be referred to herein as a “circuit,” “module,”“component,” or “system.” Furthermore, aspects of the present disclosuremay take the form of a computer program product embodied in one or moremachine-readable media having machine-readable program code embodiedthereon.

Any combination of one or more machine-readable media may be utilized.The machine-readable media may be a machine-readable signal medium or amachine-readable storage medium. A machine-readable storage medium maybe, for example, but not limited to, an electronic, magnetic, optical,electromagnetic, or semiconductor system, apparatus, or device, or anysuitable combination of the foregoing. More specific examples (anon-exhaustive list) of such a machine-readable storage medium includethe following: a hard disk, a random access memory (RAM), a read-onlymemory (ROM), an erasable programmable read-only memory (EPROM or Flashmemory), an appropriate optical fiber with a repeater, an opticalstorage device, a magnetic storage device, or any suitable combinationof the foregoing. In the context of this document, a machine-readablestorage medium may be any tangible medium that can contain, or store aprogram for use by or in connection with an instruction executionsystem, apparatus, or device, such as, for example, a microprocessor.

A machine-readable signal medium may include a propagated data signalwith machine-readable program code embodied therein, for example, inbaseband or as part of a carrier wave. Such a propagated signal may takeany of a variety of forms, including, but not limited to,electro-magnetic, optical, or any suitable combination thereof. Amachine-readable signal medium may be any machine-readable medium thatis not a machine-readable storage medium and that can communicate,propagate, or transport a program for use by or in connection with aninstruction execution system, apparatus, or device. Program codeembodied on a machine-readable signal medium may be transmitted usingany appropriate medium, including but not limited to wireless, wireline,optical fiber cable, RF signals, etc., or any suitable combination ofthe foregoing.

Computer program code for carrying out operations for aspects of thepresent disclosure may be written in any combination of one or moreprogramming languages, including object oriented programming languages,dynamic programming languages, and/or procedural programming languages.

The flowchart and block diagrams in the figures illustrate examples ofthe architecture, functionality, and operation of possibleimplementations of systems, methods and computer program productsaccording to various aspects of the present disclosure. In this regard,each block in the flowchart or block diagrams may represent a module,segment, or portion of code, which comprises one or more executableinstructions for implementing the specified logical function(s). Itshould also be noted that, in some alternative implementations, thefunctions noted in the blocks may occur out of the order illustrated inthe figures. For example, two blocks shown in succession may, in fact,be executed substantially concurrently, or the blocks may sometimes beexecuted in the reverse order, depending upon the functionalityinvolved. It will also be noted that each block of the block diagramsand/or flowchart illustration, and combinations of blocks in the blockdiagrams and/or flowchart illustration, can be implemented by specialpurpose hardware-based systems that perform the specified functions oracts, or combinations of special purpose hardware and machine-readableinstructions.

The receiver designs and techniques for receiving signals describedherein may be employed in a wide variety of different contexts. Forexample, while the receiver designs and techniques for receiving signalsdescribed herein may be employed to receive 1090 MHz Mode S ES ADS-Bmessages terrestrially and/or in space, for example from low-Earth orbitor other orbits, they also may be capable of being employed to receiveother types of signals or messages whether terrestrially or in space.

The terminology used herein is for the purpose of describing particularaspects only and is not intended to be limiting of the disclosure. Asused herein, the singular forms “a”, “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”and/or “comprising,” when used in this specification, specify thepresence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components, and/or groups thereof.

The corresponding structures, materials, acts, and equivalents of anymeans or step plus function elements in the claims below are intended toinclude any disclosed structure, material, or act for performing thefunction in combination with other claimed elements as specificallyclaimed. The description of the present disclosure has been presentedfor purposes of illustration and description, but is not intended to beexhaustive or limited to the disclosure in the form disclosed. Manymodifications and variations will be apparent to those of ordinary skillin the art without departing from the scope and spirit of thedisclosure. The aspects of the disclosure herein were chosen anddescribed in order to explain the principles of the disclosure and thepractical application, and to enable others of ordinary skill in the artto understand the disclosure with various modifications as are suited tothe particular use contemplated.

What is claimed is:
 1. A receiver for receiving 1090 MHz Mode S ExtendedSquitter (“ES”) ADS-B messages comprising: processing componentry todetermine if it is likely that a received signal includes a frequencycomponent within a range of 1090 MHz; processing componentry to compareat least part of the received signal to a model of at least part of anexpected 1090 MHz Mode S ES ADS-B message; processing componentry todetermine if it is likely that the received signal includes a 1090 MHzMode S ES ADS-B message based on a result of comparing the part of thereceived signal to the model of the part of the expected 1090 MHz Mode SES ADS-B message; processing componentry to detect multiple features ofat least part of the received signal; processing componentry to comparethe detected features of at least part of the received signal tocharacteristic features of at least part of a 1090 MHz Mode S ES ADS-Bmessage; processing componentry to determine if it is likely that thereceived signal includes a 1090 MHz Mode S ES ADS-B message based on aresult of comparing the detected features of at least part of thereceived signal to characteristic features of at least part of a 1090MHz Mode S ES ADS-B message; and processing componentry to implement anon-coherent filter configured to recover a 1090 MHz Mode S ES ADS-Bmessage from the received signal following a determination that it islikely that the received signal includes a frequency component within arange of 1090 MHz, a determination that it is likely that the receivedsignal includes a 1090 MHz Mode S ES ADS-B message based on the resultof comparing the part of the received signal to the model of the part ofthe expected 1090 MHz Mode S ES ADS-B message, and a determination thatit is likely that the received signal includes a 1090 MHz Mode S ESADS-B message based on the result of comparing the detected features ofat least part of the received signal to characteristic features of atleast part of a 1090 MHz Mode S ES ADS-B message.
 2. The receiver ofclaim 1, wherein the processing componentry to determine if it is likelythat the received signal includes a frequency component within a rangeof 1090 MHz comprises processing componentry to: perform a Fouriertransform to at least a portion of the received signal to generate afrequency domain representation of the received signal; and determine ifa frequency component of the frequency domain representation of thereceived signal is within a range of 1090 MHz.
 3. The receiver of claim1, wherein: the processing componentry to compare at least part of thereceived signal to a model of at least part of an expected 1090 MHz ModeS ES ADS-B message comprises processing componentry to calculate across-correlation between the part of the received signal and the modelof the part of the expected 1090 MHz Mode S ES ADS-B message; and theprocessing componentry to determine if it is likely that the receivedsignal includes a 1090 MHz Mode S ES ADS-B message based on a result ofcomparing the part of the received signal to the model of the part ofthe expected 1090 MHz Mode S ES ADS-B message comprises processingcomponentry to determine if it is likely that the received signalincludes a 1090 MHz Mode S ES ADS-B message based on the calculatedcross-correlation between the part of the received signal and the modelof the part of the expected 1090 MHz Mode S ES ADS-B message.
 4. Thereceiver of claim 3, wherein: the processing componentry to calculate across-correlation between the part of the received signal and the modelof the part of the expected 1090 MHz Mode S ES ADS-B message comprisesprocessing componentry to calculate a cross-correlation between the partof the received signal and a model of an expected pulse pattern of apreamble of a 1090 MHz Mode S ES ADS-B message; and the processingcomponentry to determine if it is likely that the received signalincludes a 1090 MHz Mode S ES ADS-B message based on the calculatedcross-correlation between the part of the received signal and the modelof the part of the expected 1090 MHz Mode S ES ADS-B message comprisesprocessing componentry to determine if it is likely that the receivedsignal includes a 1090 MHz Mode S ES ADS-B message based on thecalculated cross-correlation between the part of the received signal andthe model of the expected pulse pattern of the preamble of a 1090 MHzMode S ES ADS-B message.
 5. The receiver of claim 3, wherein: theprocessing componentry to calculate a cross-correlation between the partof the received signal and the model of the part of the expected 1090MHz Mode S ES ADS-B message comprises processing componentry tocalculate a cross-correlation between the part of the received signaland a model of an expected pulse pattern of a preamble and at least thefirst five bit periods of a 1090 MHz Mode S ES ADS-B message; and theprocessing componentry to determine if it is likely that the receivedsignal includes a 1090 MHz Mode S ES ADS-B message based on thecalculated cross-correlation between the part of the received signal andthe model of the part of the expected 1090 MHz Mode S ES ADS-B messagecomprises processing componentry to determine if it is likely that thereceived signal includes a 1090 MHz Mode S ES ADS-B message based on thecalculated cross-correlation between the part of the received signal andthe model of the expected pulse pattern of the preamble and the at leastfirst five bit periods of a 1090 MHz Mode S ES ADS-B message.
 6. Thereceiver of claim 1, wherein: the processing componentry to detectmultiple features of the part of the received signal comprisesprocessing componentry to detect a measure of amplitude consistency ofpulses within the part of the received signal; the processingcomponentry to compare the detected features of the part of the receivedsignal to characteristic features of at least part of a 1090 MHz Mode SES ADS-B message comprises processing componentry to compare thedetected measure of amplitude consistency of pulses within the part ofthe received signal to a characteristic amplitude consistency for pulseswithin at least part of a 1090 MHz Mode S ES ADS-B message; andprocessing componentry to determine if it is likely that the receivedsignal includes a 1090 MHz Mode S ES ADS-B message based on a result ofcomparing the detected features of the part of the received signal tocharacteristic features of at least part of a 1090 MHz Mode S ES ADS-Bmessage comprises processing componentry to determine if it is likelythat the received signal includes a 1090 MHz Mode S ES ADS-B messagebased on a result of comparing the detected measure of amplitudeconsistency of pulses within the part of the received signal to thecharacteristic amplitude consistency for pulses within at least part ofa 1090 MHz Mode S ES ADS-B message.
 7. The receiver of claim 1, wherein:the processing componentry to detect multiple features of the part ofthe received signal comprises processing componentry to detect a measureof phase consistency of the part of the received signal; the processingcomponentry to compare the detected features of the part of the receivedsignal to characteristic features of at least part of a 1090 MHz Mode SES ADS-B message comprises processing componentry to compare thedetected measure of phase consistency of the part of the received signalto a characteristic phase consistency for at least part of a 1090 MHzMode S ES ADS-B message; and the processing componentry to determine ifit is likely that the received signal includes a 1090 MHz Mode S ESADS-B message based on a result of comparing the detected features ofthe part of the received signal to characteristic features of at leastpart of a 1090 MHz Mode S ES ADS-B message comprises processingcomponentry to determine if it is likely that the received signalincludes a 1090 MHz Mode S ES ADS-B message based on a result ofcomparing the detected measure of phase consistency of the receivedsignal to the characteristic phase consistency for at least part of a1090 MHz Mode S ES ADS-B message.
 8. The receiver of claim 1, wherein:the processing componentry to detect multiple features of the part ofthe received signal comprises processing componentry to detect a measureof residual phase error of the part of the received signal; theprocessing componentry to compare the detected features of the part ofthe received signal to characteristic features of at least part of a1090 MHz Mode S ES ADS-B message comprises processing componentry tocompare the detected measure of residual phase error of the part of thereceived signal to a characteristic residual phase error for at leastpart of a 1090 MHz Mode S ES ADS-B message; and the processingcomponentry to determine if it is likely that the received signalincludes a 1090 MHz Mode S ES ADS-B message based on a result ofcomparing the detected features of the part of the received signal tocharacteristic features of at least part of a 1090 MHz Mode S ES ADS-Bmessage comprises processing componentry to determine if it is likelythat the received signal includes a 1090 MHz Mode S ES ADS-B messagebased on results of comparing the detected measure of residual phaseerror of the part of the received signal to the characteristic residualphase error for at least part of a 1090 MHz Mode S ES ADS-B message. 9.The receiver of claim 1, wherein: the processing componentry to detectmultiple features of the part of the received signal comprisesprocessing componentry to detect measures of amplitude consistency ofpulses within the part of the received signal, phase consistency of thepart of the received signal, and residual phase error of the part of thereceived signal; the processing componentry to compare the detectedfeatures of the part of the received signal to characteristic featuresof at least part of a 1090 MHz Mode S ES ADS-B message comprisesprocessing componentry to compare: the detected measure of amplitudeconsistency of pulses within the part of the received signal to acharacteristic amplitude consistency for pulses within at least part ofa 1090 MHz Mode S ES ADS-B message, the detected measure of phaseconsistency of the part of the received signal to a characteristic phaseconsistency for at least part of a 1090 MHz Mode S ES ADS-B message, andthe detected measure of residual phase error of the part of the receivedsignal to a characteristic residual phase error for at least part of a1090 MHz Mode S ES ADS-B message, and the processing componentry todetermine if it is likely that the received signal includes a 1090 MHzMode S ES ADS-B message based on a result of comparing the detectedfeatures of the part of the received signal to characteristic featuresof at least part of a 1090 MHz Mode S ES ADS-B message comprisesprocessing componentry to determine if it is likely that the receivedsignal includes a 1090 MHz Mode S ES ADS-B message based on results ofcomparing the detected measure of amplitude consistency of pulses withinthe part of the received signal to the characteristic amplitudeconsistency for pulses within at least part of a 1090 MHz Mode S ESADS-B message, the detected measure of phase consistency of the part ofthe received signal to the characteristic phase consistency for at leastpart of a 1090 MHz Mode S ES ADS-B message, and the detected measure ofresidual phase error of the part of the received signal to thecharacteristic residual phase error for at least part of a 1090 MHz ModeS ES ADS-B message.
 10. The receiver of claim 1, wherein: the processingcomponentry to detect multiple features of at least part of the receivedsignal comprises: processing componentry to detect n features of atleast part of the received signal, and processing componentry togenerate a feature vector comprising n elements representing the ndetected features of the part of the received signal; and the processingcomponentry to compare the detected features of the part of the receivedsignal to characteristic features of at least part of a 1090 MHz Mode SES ADS-B message comprises: processing componentry to project thefeature vector into a feature space comprising n dimensions and thatincludes representations of at least parts of characteristic 1090 MHzMode S ES ADS-B messages, and processing componentry to compare theprojected feature vector to the representations of the parts of thecharacteristic 1090 MHz Mode S ES ADS-B messages in the feature space.11. The receiver of claim 1, wherein the processing componentry toimplement a non-coherent filter configured to recover a 1090 MHz Mode SES ADS-B message from the received signal comprises processingcomponentry to implement a non-coherent filter configured to recover a1090 MHz Mode S ES ADS-B message from the received signal without usinginformation about the phase of the 1090 MHz Mode S ES ADS-B message. 12.The receiver of claim 1, wherein the processing componentry to implementa non-coherent filter configured to recover a 1090 MHz Mode S ES ADS-Bmessage from the received signal comprises processing componentry toimplement a non-coherent filter with an impulse response that is a 0.5μsec 1090 MHz pulse.
 13. The receiver of claim 1, wherein the receiveris configured to be integrated with a satellite and to receive a 1090MHz Mode S ES ADS-B message in orbit around the Earth.
 14. The receiverof claim 1, wherein the receiver is configured to receive a 1090 MHzMode S ES ADS-B message from a terrestrial position.
 15. The receiver ofclaim 1, wherein the processing componentry to implement a non-coherentfilter configured to recover a 1090 MHz Mode S ES ADS-B message from thereceived signal comprises processing componentry to recover a 1090 MHzMode S ES ADS-B message from the received signal when the E_(b)/N_(o) ofthe received signal is not more than 10 dB.
 16. A receiver for receiving1090 MHz Mode S Extended Squitter (“ES”) ADS-B messages comprising:means for determining if it is likely that a received signal includes afrequency component within a range of 1090 MHz; means for comparing atleast part of the received signal to a model of at least part of anexpected 1090 MHz Mode S ES ADS-B message; means for determining if itis likely that the received signal includes a 1090 MHz Mode S ES ADS-Bmessage based on a result of comparing the part of the received signalto the model of the part of the expected 1090 MHz Mode S ES ADS-Bmessage; means for detecting multiple features of at least part of thereceived signal; means for comparing the detected features of at leastpart of the received signal to characteristic features of at least partof a 1090 MHz Mode S ES ADS-B message; means for determining if it islikely that the received signal includes a 1090 MHz Mode S ES ADS-Bmessage based on a result of comparing the detected features of at leastpart of the received signal to characteristic features of at least partof a 1090 MHz Mode S ES ADS-B message; and means for implementing anon-coherent filter configured to recover a 1090 MHz Mode S ES ADS-Bmessage from the received signal following a determination that it islikely that the received signal includes a frequency component within arange of 1090 MHz, a determination that it is likely that the receivedsignal includes a 1090 MHz Mode S ES ADS-B message based on the resultof comparing the part of the received signal to the model of the part ofthe expected 1090 MHz Mode S ES ADS-B message, and a determination thatit is likely that the received signal includes a 1090 MHz Mode S ESADS-B message based on the result of comparing the detected features ofat least part of the received signal to characteristic features of atleast part of a 1090 MHz Mode S ES ADS-B message.
 17. The receiver ofclaim 16, wherein: the means for comparing at least part of the receivedsignal to a model of at least part of an expected 1090 MHz Mode S ESADS-B message comprises means for calculating a cross-correlationbetween the part of the received signal and a model of an expected pulsepattern of a preamble of a 1090 MHz Mode S ES ADS-B message; and themeans for determining if it is likely that the received signal includesa 1090 MHz Mode S ES ADS-B message based on a result of comparing thepart of the received signal to the model of the part of the expected1090 MHz Mode S ES ADS-B message comprises means for determining if itis likely that the received signal includes a 1090 MHz Mode S ES ADS-Bmessage based on the calculated cross-correlation between the part ofthe received signal and the model of the expected pulse pattern of thepreamble of a 1090 MHz Mode S ES ADS-B message.
 18. The receiver ofclaim 16, wherein: the means for comparing at least part of the receivedsignal to a model of at least part of an expected 1090 MHz Mode S ESADS-B message comprises means for calculating a cross-correlationbetween the part of the received signal and a model of an expected pulsepattern of a preamble and at least the first five bit periods of a 1090MHz Mode S ES ADS-B message; and the means for determining if it islikely that the received signal includes a 1090 MHz Mode S ES ADS-Bmessage based on a result of comparing the part of the received signalto the model of the part of the expected 1090 MHz Mode S ES ADS-Bmessage comprises means for determining if it is likely that thereceived signal includes a 1090 MHz Mode S ES ADS-B message based on thecalculated cross-correlation between the part of the received signal andthe model of the expected pulse pattern of the preamble and the at leastfirst five bit periods of a 1090 MHz Mode S ES ADS-B message.
 19. Thereceiver of claim 16, wherein the receiver is configured to beintegrated with a satellite and to receive a 1090 MHz Mode S ES ADS-Bmessage in orbit around the Earth.
 20. A method for recovering a 1090MHz Mode S Extended Squitter (“ES”) ADS-B message from a received signalcomprising: determining if it is likely that a received signal includesa frequency component within a range of 1090 MHz; comparing at leastpart of the received signal to a model of at least part of an expected1090 MHz Mode S ES ADS-B message; determining if it is likely that thereceived signal includes a 1090 MHz Mode S ES ADS-B message based on aresult of comparing the part of the received signal to the model of thepart of the expected 1090 MHz Mode S ES ADS-B message; detectingmultiple features of at least part of the received signal; comparing thedetected features of at least part of the received signal tocharacteristic features of at least part of a 1090 MHz Mode S ES ADS-Bmessage; determining if it is likely that the received signal includes a1090 MHz Mode S ES ADS-B message based on a result of comparing thedetected features of at least part of the received signal tocharacteristic features of at least part of a 1090 MHz Mode S ES ADS-Bmessage; and implementing a non-coherent filter configured to recover a1090 MHz Mode S ES ADS-B message from the received signal following adetermination that it is likely that the received signal includes afrequency component within a range of 1090 MHz, a determination that itis likely that the received signal includes a 1090 MHz Mode S ES ADS-Bmessage based on the result of comparing the part of the received signalto the model of the part of the expected 1090 MHz Mode S ES ADS-Bmessage, and a determination that it is likely that the received signalincludes a 1090 MHz Mode S ES ADS-B message based on the result ofcomparing the detected features of at least part of the received signalto characteristic features of at least part of a 1090 MHz Mode S ESADS-B message.
 21. A receiver for receiving 1090 MHz Mode S ExtendedSquitter (“ES”) ADS-B messages comprising: processing componentry todetermine if it is likely that a received signal includes a 1090 MHzMode S ES ADS-B message based on a comparison of the received signalagainst expected characteristics of a model 1090 MHz Mode S ES ADS-Bmessage, wherein the comparison comprises determining whether a pulsepattern of the received signal conforms to an expected pulse pattern ofa 1090 MHz Mode S ES ADS-B message, wherein the expected pulse patternincludes a preamble section followed by a downlink format section, andwherein the expected pulse pattern for the preamble section and downlinkformat section are defined by an expected number of pulses over arespective period for each section; and a non-coherent filter configuredto recover a 1090 MHz Mode S ES ADS-B message from the received signalfollowing a determination that it is likely that the received signalincludes a 1090 MHz Mode S ES ADS-B message.